1. Field of the Invention
The present invention relates to wireless telecommunications and, more particularly, to methods and systems for frequency usage in a wireless telecommunications network.
2. Description of Related Art
As wireless telecommunications have become increasingly popular, wireless telecommunications networks have been increasing their wireless capacity in order to accommodate the increase in wireless traffic. Whether a wireless telecommunications network uses an analog format, such as AMPS, or a digital format, such as TDMA or CDMA, the wireless capacity of the network is typically limited by a number of factors, including the bandwidth of the available frequency spectrum.
For example, as specified in IS-95, a CDMA air interface includes a 1.23 MHz wide downlink band for communications in the forward direction, i.e., towards the mobile station and another 1.23 MHz wide uplink band for communications in the reverse direction, i.e., from the mobile station. The same 1.23 MHz bands may be used in up to 512 wireless coverage areas, such as cells or sectors, without interference, by using a distinct pseudonoise (PN) code offset in each wireless coverage area. Within each wireless coverage area, the downlink band may accommodate up to nine (9) control channels, e.g., a pilot channel, a sync channel, up to seven (7) paging channels, and up to sixty-one (61) traffic channels, identified by distinct Walsh codes. The uplink band in each wireless coverage area may accommodate up to thirty-two (32) access channels and sixty-two (62) traffic channels, identified by distinct PN codes. Other CDMA specifications, such as IS-2000 may support different channels and different bandwidths. Nonetheless, if all of a wireless coverage area's limited number of traffic channels are being used, that wireless coverage area may be unable to support additional calls. As a result, in high-traffic wireless coverage areas, such as airports, wireless service may be unreliable during times of peak usage.
One approach for adding wireless capacity is to make additional frequency spectrum available to high-traffic wireless coverage areas. However, if the additional frequency spectrum is used only in the high-traffic wireless coverage area, then calls may be dropped when mobile stations using the additional frequency spectrum move to other wireless coverage areas that do not support the additional frequency spectrum. In particular, if a mobile station is engaged in a call on a given frequency and then moves into a wireless coverage area that does not support that frequency, a “hard handoff” process occurs. A hard handoff process is a “break before make” process in which an existing communication link is dropped before a new one is made. Thus, hard handoffs, such as inter-frequency handoffs, often result in dropped calls.
In contrast, CDMA supports a “soft handoff” process for mobile stations that move from one wireless coverage to another using the same frequency. A soft handoff process is a “make before break” process in which a new communication link is made before the existing communication link is dropped. As a result, soft handoffs are much less likely to cause calls to be dropped than hard handoffs.
Accordingly, one approach for minimizing dropped calls when additional frequency spectrum is added to a high-traffic wireless coverage area is to make the additional frequency spectrum available in surrounding wireless coverage areas as well. In this way, a mobile station using the additional frequency spectrum in the high-traffic wireless coverage area may use a soft handoff process to move into an adjoining wireless coverage that supports the additional frequency spectrum. However, adding frequency spectrum to surrounding wireless areas that may not themselves need additional wireless capacity is inefficient and substantially more costly than adding wireless capacity to only the high-traffic wireless coverage areas.
Accordingly, there is a need to increase wireless capacity in high-traffic wireless coverage areas in an efficient manner that minimizes disruptions to wireless service.